I have an ARC LS 25 MK 1.........

Rwd:
I am using the VT100 MKII with my LS 25 MKI also. The combo with the NOS is awesome. A set of four 6h23 NOS will cost about $100 ($25 each)

I am running my PH3SE into my LS2B Mk II. For now, I have a matched set and a single (total of 3) Amperex 7308 white label (circa 1965) in my PH3, and a Mullard 10M 6DJ8 in my LS2. If you are interested in finding out more about my system, you can see it listed under FrankC on Audio Asylum's member systems listing.

For cheaper alternatives, some other ARC LS25 and PH3 SE owners have recommended the Russian 6H23N EB for the PH3 SE and Amperex Orange Globe 6DJ8 for the LS25.

The Amperex Orange Globe has a Orange color Globe logo on the tube. There are two types. The pre-1970 design has a ring inside, while the later version has an "A" frame. See JoeS' excellent post on tubes of this family on the Audio Asylum site.